Platelets play a central role in hemostasis and thromboembolic disease, in part adherence to each other (aggregation) or vessel walls (adhesion). These reactions are mediated in part by interactions with a class of large disulfide-linked glycoproteins. We have found that thrombin induces specific and saturable binding of one of these glycoproteins, fibronectin (fn) to platelets. Based on our preliminary data, we envision the interaction of fn with the platelet to involve the four steps diagrammed below. Resting Cell greater than FN Binding Site II greater than Fn Binding III greater than Fn Processing IV greater than Functional. We will dissect and define each of these individual steps in the reaction sequence. Reaction I describes the induction of the fn binding site by platelet stimulation. We shall identify stimuli which do and do not support fn binding using both secretory and non secretory stimuli. The effect of selected drugs on fn binding will be investigated as a means of defining biochemical pathways required for binding site induction. Reaction II entails fn binding to the platelet. The effect of environmental conditions such as temperature, pH and divalent ions, will be established, and the domains of fn which are recognized by the platelet will be defined. In addition, multiple approaches will be used to identify the binding site for fn. Particular emphasis will be given to the role of fibrin(ogen) in fn binding based on our preliminary data. Reaction III entails processing of platelet-bound fn, and we shall investigate cross-linking of fn on the platelet surface via transglutaminases or disulfide bonding, internalization of fn, and interactions with cytoskeletal elements. Reaction IV involves the functional consequences of fn binding to the platelet, either as a result of initial binding or subsequent processing. Its effect on platelet aggregation and adhesion, clot retraction and monocyte-platelet interaction represent likely candidate events which will be explored.